[docs] [issue8925] Improve c-api/arg.rst: use "bytes" or "str" types instead of "string"

[STINNER Victor]

STINNER Victor <victor.stinner at haypocalc.com> added the comment:

> Is there a reST construct to use here, say to reference a PEP 
> or point to a glossary entry that defines this protocol?

buffer API ("protocol" ?) is documented in Doc/c-api/buffer.rst. Should I link to this document? How?

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